igneous

Basalt

Basalt (Mafic Extrusive Igneous Rock)

Hardness: 6 (Mohs scale); Color: Dark grey to black or dark greenish-brown; Luster: Dull (aphanitic texture); Crystal Structure: Fine-grained, often containing microscopic plagioclase feldspar and pyroxene; Cleavage: None/Irregular fracture; Specific Gravity: 2.8–3.0.

Hardness: 6 (Mohs scale)
Color: Dark grey to black or dark greenish-brown
Luster: Dull (aphanitic texture)

Identified March 23, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava (mafic lava) exposed at or very near the Earth's surface. Can range from modern eruptions to billions of years old; found in oceanic crust and volcanic hotspots.

Uses & applications

Extensively used in construction as aggregate for road base, concrete, and railroad ballast. Also used as a dimension stone for floor tiles and in the production of stone wool insulation.

Geological facts

Basalt is the most common rock type in the Earth's crust, making up most of the ocean floor. It is also common on other planetary bodies like the Moon (lunar maria), Mars, and Venus.

Field identification & locations

Identify in the field by its dark color, heavy weight (density), and lack of visible large crystal grains. Commonly found in riverbeds (where it becomes rounded, as seen in the image) or near volcanic sites. Collectors look for vesicular basalt (with gas bubble holes) or specimens containing green olivine crystals.

Other igneous specimens

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

mineral

Dark Igneous Rock (Likely Basalt or Diabase)

Basalt (extrusive igneous rock) or Diabase (intrusive igneous rock - also known as Dolerite), largely composed of plagioclase feldspar, pyroxene (augite), and sometimes olivine and amphibole. Exact mineralogy would require thin section analysis.

Igneous